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CAT5 tester?

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New Member
First, a little background, I'm not an electronics engineer, but I've always been fascinated with electronics and wish I knew more (and had the time to learn...) The other day, I built a long CAT5 cable to run to the other room (no wireless on this one laptop...) and when I plugged it in, it wasn't working. So I looked up prices for CAT5 testers and was pretty much blown away, no way I'm buying one of those for the few cables I do make. (And anyway, it'll be a whole lot more fun to build something....) So here's the thought, make a tester myself that just checks each of the four pairs for continuity (yeah, I know only two are used in my current network, but CAT6 uses all four(I think?). And since it's a pet project, I can embellish it and make it "cool." (A bunch of LED's flashing in sequence, that can be either red or green qualifies as "cool" :D) First, the "remote" end is just a loopback, pins 1&2, 3&6, 4&5 and 7&8 are just tied together, and enclosed in a small box. I'll figure out a way to attach it to the main unit (velcro it to the side?), but it can be detached and connected in another room (for when I do eventually drop those two other network access points).

Then, in one of my old Radio Shack engineer notebooks, I found a circuit for a 1-of-4 sequencer:


Great!! Each of those circuits can then test each of the pairs in the CAT5. Next, I just put together the logic (a couple of AND gates (4081) and an XOR gate (4070)) to test each pair and visually provide feedback with a RED/GREEN (dual) LED, green for a good connection, red for an open:


Now, this is where my question starts... Is this going to work, without problems? Assume the circuits will be replicated three more times for the other three pairs, three more LEDs, etc... I'm going to assume that I only need a resistor for all four LEDs, is this a correct assumption, or should each have its own? Decoupling capacitors, that's something I don't know much about (I know why and what they do, I just don't know enough about when and where to apply them), is/are one or two needed? Since these are CMOS logic gates, should there be a transistor in there for each LED so the gate isn't driving it directly? Also, would it be better to use TTL IC's instead, especially since I'll probably be connecting/disconnecting cables while there's power going to the circuit. Also, then the TTL (would?) be able to drive the LED's directly without a transistor and not worry so much about static? (Of course, I would look at using 4.5v instead of 6v)...

Are there other ways you would do this? Or in other words, what would you do to change it? I am wide open to any and all suggestions...


Well-Known Member
It looks like the 555 timer oscillator is operating at a relatively low frequency.

It might operate slightly better then an ohm meter check.

Besides opens, the most other common degradation on Cat5 is a kink in the line. This causes the twisted pair transmission lines to inter-mingle creating cross-talk between the pairs.

Cat6 cable has a plastic 'X' spacer within the cable to do a better job of keeping the four twisted pairs physically separated in their own little quadrant of the cable.


New Member
Circuit looks like it does what you expect. Your tester will find opens but probably not short circuits or miswires (crossover instead of straight). I confess, in my experience, most of my cat5 errors have been opens, failiing to insert far enough or crimp.

If you drive every pair as differential, and put a bidirectional LED across each pair at the other end, it will tell you if the cable is wired correctly.


New Member
It should detect miswires since the "remote" is just looping the signal back (tying each transmit and receive pair together for a straight-through patch cable). But you're right, it wouldn't detect a short, but assuming I'm using new cabling, that shouldn't be an issue if all I'm doing is putting new jacks on the end(?). I'd rather not put LEDs on the remote, though that would make an already "cool" project even cooler (more LEDs is always, inherently good, per the law of the universe...)

I haven't tested any of the circuits yet, but I'm expecting to get the LEDs to flash, in sequence, about one per second. So yeah, it should be a relatively slow oscillation.

My main questions are revolving around CMOS vs. TTL. With this, should I have to worry about static and thus limit me to TTL? And for the LED current, will one type over the other work better (use less devices, can drive the LED without transistors, etc...)?

Than again, if I went with LEDs in the remote, I'd look at using a 4017 (Decade Counter)... But that would make things interesting determining when an open occurs and flashing the red LED vs. the green. I'll think on that for a while, I can probably use a missing pulse detector and some logic to take care of that.

search Ebay for "Cable Tester LAN USB Network RJ45 Cat5 RJ11"

I bought one for $3.00 with free shipping. It works good. It does RJ45, RJ11, and USB cables. Even has a remote for long cable runs. Enclosed in a box, it runs on a 9V battery.

The only reason to build one is just for the experience. I'll bet you can't build one for that price.


New Member
Well, JohnSmith... When I did my serch, I was finding them anywhere from $20 to over $100, not thinking to look at ebay (had a few bad experiences...)

Anyway, that may be my answer to this one. Thanks...

Though it would have been cool, you're right, no way I could have built anything like that for this cheap...
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